The overall goal of this proposal is to examine the hypothesis that spinal recurrent inhibition regulates activity across motoneuron pools that receive common commands during centrally generated activities like locomotion and scratching. Recent work from this laboratory suggests that common patterns of motor activity during locomotion determine the distribution of recurrent inhibition. The hypothesis stated above has been formulated to integrate these findings with other important features of the organization of recurrent inhibition. There are three specific aims. The first specific aim is to compare the organization of recurrent inhibition and Ia reciprocal inhibition between motoneuron pools that control the ankle. Aim-1 studies will provide data necessary to provide a rigorous comparison of the organization of recurrent inhibition and Ia reciprocal inhibition to patterns of motor activity and the organization of centrally generated motor commands. This information will be used in testing the hypothesis for the second specific aim. The second specific aim is to test the hypothesis that motoneuron pools that are interconnected by recurrent inhibition receive common motor commands during centrally generated motor activities. In these studies, crosscorrelation and coherence functions between motor patterns of selected hindlimb motoneuron pools will be examined during fictive locomotion and fictive scratching. Motoneuron pools that receive common motor commands, as indicated by the crosscorrelation and coherence functions, will be compared to patterns of recurrent inhibition and Ia reciprocal inhibition between motoneuron pools. The third specific aim is to test the hypothesis that recurrent inhibition tunes motor output during centrally generated motor activities by directing motor commands to the most active motor pools. The amplitudes and ratios of Ia reflex responses in different motor pools will be examined during fictive locomotion and scratching in the Aim-3 studies. These values will be evaluated in relation to known patterns of recurrent inhibition and monosynaptic Ia projections. They will also be compared following the application of cholinergic antagonists to block recurrent inhibition. The health related significance of this proposal is that these studies will provide information about the regulation by recurrent inhibition of the relative degree of activity in motoneuron pools. This spinal circuit is subject to control from supraspinal centers, which may be interrupted in disease and spinal injury, with attendant deterioration in the regulation of motor activity.